It is conventional to use safety melting fuses in the electrical systems of motor vehicles for protecting the electrical components. Providing protection with such melting fuses has the disadvantage that an optimal protection of the circuit system and its components is not possible due to several factors. Normally, conventional electrical conductors in electrical systems for motor vehicles are capable of withstanding transient electrical excess currents that are higher than the blow out current of the fuse provided that the excess currents have a short time duration. Thus, for transient short duration excess currents a conventional fuse is not accurately dimensioned. On the other hand, when excess current have a longer duration, such a fuse tends to interrupt the circuit too late. In that case, the electrical conductor and/or circuit component is not sufficiently protected. For example, if an excess current is 35% relative to the rated fuse blow out threshold of the fuse it may take half an hour until the fuse actually interrupts the circuit. Even at an excess current of 250% of the rated trigger current of the fuse, it may take 5 seconds until the fuse interrupts the circuit.
In connection with so-called passive melting fuses there are several conventional methods to influence the response characteristic of such fuses. On the one hand, different materials for making the fuse are used, such as copper or zinc forming the melting elements. On the other hand, the melting zones of such fuses may be covered with tin in order to influence or adapt the response characteristic. These methods have the disadvantage that a substantial effort and expense is involved because each different electrical conductor system requires a new adaptation for achieving an optimal response characteristic. The reason for requiring this adaptation is the fact that not only the characteristics of the melting elements must be taken into account, but also individual conditions of a particular electrical conductor system must receive attention such as damaged conductor cross-sections, operating temperatures, and faulty insulations which all have an influence on the response characteristic. As a result, even with a high effort and expense only a limited adaptation of the fuses to the particular electrical system can be achieved by the above mentioned conventional methods. As a result, standardized fuses are used in electrical systems. Such standardized fuses are cost effective, but provide only a limited protection.
German Patent Publication DE 195 27 997 A1 discloses a method in which the current passing through the fuse is measured. If the measured current exceeds a predetermined tripping value, an active blowing out of the fuse is performed. A disadvantage of this conventional method is seen in that the blow out current value or characteristic of the fuse is fixed, whereby the conductor capabilities to withstand certain overloads for short time durations are not utilized or not fully utilized.
German Patent Publication DE 44 45 060 C1 discloses a power switch equipped with an electronic circuit breaker for processing adjustable parameters, particularly the tripping current and the delay time. A bypass circuit (15) causes an enforced opening of the power switch in response to a situation in which the switch did not open even though the adjusted tripping current was exceeded. The bypass circuit (15) includes circuit elements for forming a time and current dependent response characteristic, whereby the protection against the destruction of the power switch is improved. The response characteristic of the bypass circuit (15) may be automatically variable depending on the parameters that have been adjusted for a normal tripping and opening of the power switch. It is a disadvantage of such an arrangement that the bypass circuit is expensive and does not itself serve for interrupting the circuit, but rather merely protects the power switch against destruction when the electronic circuit breaker fails.
A handbook entitled "Hilfsbuch der Elektrotechnik", published by AEG Telefunken VOl. 2, 11th Edition, Berlin, 1979, pgs. 294 to 391, discloses protection devices particularly over current or excess current time relays which have a release timing dependent on the size of the over or excess current, whereby the release or tripping characteristic of the over current time relay corresponds to the load characteristic of the circuit arrangement to be protected. A disadvantage in such an arrangement is the fact that the entire load characteristic must be recorded and stored in a memory. Additionally, when measuring the excess or over current, a time duration must be measured, which is then compared with the tripping time duration. Such an approach requires a substantial effort and expense.